Peculiarities of structure formation of corrosion-resistant martensitic steel of 09Kh16N4BL grade

It is shown that, all other things being equal, the smelting of foundry corrosion-resistant chromium-nickel steel of the martensitic class in an open induction furnace with deoxidation at the outlet of the melt with REM and zirconium provides low values of total contamination with non-metallic inclusions of melting at the level of 0.98×10-4 determined by the method “L2“ GOST 1778-2022. At the same time, the total content of rare earth (La, Ce, Nd) and surfactant elements (B, Zr, S) in the composition of non-metallic inclusions reaches 75 % (by weight), and the inclusions themselves exhibit an increased tendency to coalescence. The lines of the carbide phase with a length of 4.3 to 78.4 microns found during electron microscopic and metallographic studies located in the interbranch zones solidifying at the last stages of crystallization belong to primary and secondary niobium carbides, formed respectively when niobium is introduced into the melt and as a result of the eutectic reaction occurring at the final stage of solidification. Qualitative patterns of dendritic liquation of components for high-alloyed martensitic steels have not yet been found, however, for the compositions studied in the work, it was found that Sg, Ni, Si, Si, Mo and Mp during crystallization liquify directly and enrich the interbranch zones forming a solid solution with an increased content of carbide-forming elements. At the same time, at the stage of dendritic crystallization, due to the adsorption of active additive surfactants (S, Sn, B, Pb, Se, Sb, the total volume of which exceeded 0.09 % (wt.), an insurmountable barrier is formed to realize the composition-leveling diffusion redistribution of elements between the axes of dendritic branches and the interendritic space and, as a result, the observed even greater heterogeneity of the structure under any thermal influences. Metallographic studies of cast samples revealed abnormal «white» zones in their volume with an increased content of all alloying elements relative to the zone of regular crystallization. An intermetallic compound based on the Fe-Cr-Mo system (average composition % (wt.): 49.61 Fe, 27.33 Cr, 16.41 Mo, 2.59 Mn, 2.08 Si, 1.29 V, 0.69 Nb) with inclusions of ultrafine cubic primary niobium carbides was recorded in the volumes of the interbranches of the «white» zones crystallized at the last stages in volume. It is shown that niobium, unlike other elements, exhibits reverse liquation in the «white» zone, i. e., during crystallization it accumulates in dendrites, however, in the zone of regular crystallization, niobium liquates directly, enriching the interbranches.